Conventionally, a tubular or a flat film-shaped packaging material has been sealed by heat sealing (lock sealing) so as to protect the contents contained therein during storage, during transport, or the like. Regarding the improvement of the seal strength in the lock sealing, various investigations have been carried out into aspects such as the packaging material, the packaging machine, or the packaging conditions. As a polymer material used for the packaging material, for example, ethylene-based polymers such as polyethylene or an ethylene-vinyl acetate copolymer are known. Above all, an ionomer is widely used, since the ionomer is excellent in terms of a hot tack property, a low-temperature sealing property, oil resistance, and the like.
Further, a method of heat sealing (peelable sealing) has also been conducted, which makes it possible to protect the contents with a strong lock sealing property to a certain degree, and to open the package by peeling the sealed portion while applying force when it is desired to take the contents out of the packaging material.
For example, a polymer material including 80% by weight to 93% by weight of an ethylene/acid ionomer and 7% by weight to 20% by weight of a propylene/α-olefin copolymer has been disclosed (see, for example, Japanese Patent Application Publication (JP-B) No. 1-49382). It is said that the lock sealing and the peelable sealing can be performed on a packaging material that uses the polymer material, depending on the temperature. When a film or a layered material is formed using the polymer material, and filled and packed using a packaging machine, by lock-sealing the backing or bottom part under a high temperature condition and peelable-sealing the top side under a low temperature condition, it is possible to perform packaging having both a lock seal portion and a peelable seal portion by using a single packaging material. As the polymer material to be used for such packaging, for example, a layered packaging material is known, which is obtained by layering a base material layer, such as a polyester layer, and an ionomer layer via an adhesion layer. According to the relationship between the sealing temperature and the seal strength, the layered packaging material as described above is required to exhibit a certain level of seal strength suitable for peelable sealing under a low temperature sealing condition, and to exhibit a high seal strength suitable for lock sealing under a high temperature sealing condition.
Further, in packaging materials, practically, it is necessary to stably carry out the lock sealing and the peelable sealing. Specifically, it is required for packaging materials to have a temperature region in which the peelable sealing can be satisfactorily performed, that is, a temperature region (a plateau region) in which the seal strength of a peelable seal portion does not change significantly depending on the temperature.
As a technique relating to such a sealing form, a layered film for packaging which is provided with a seal layer of a polymer composition including 60 parts by weight to 95 parts by weight of a metal salt of an ethylene-α,β-unsaturated carboxylic acid-based copolymer and 40 parts by weight to 5 parts by weight of an ethylene-α,β-unsaturated carboxylic acid ester copolymer has been disclosed (see, for example, JP-B No. 5-11549). It is said that the layered film for packaging exhibits a relatively low and constant seal strength in a wide temperature range (namely, a plateau region is exhibited), and also exhibits a high seal strength under a high temperature sealing condition.
Further, a blend of a propylene copolymer and an ionomer having 5% by weight to 25% by weight of a unit derived from isobutyl acrylate, in addition to a unit derived from ethylene and a unit derived from an acid, has been disclosed (see, for example, JP-B No. 1-49382).
Furthermore, as a resin composition by which a high seal strength is obtained in a high temperature region at the time of lock sealing (for example, 20 N/15 mm or more at 180° C. or higher) and, at the time of peelable sealing, a seal strength being equal to or higher than heretofore is stably obtained, while suppressing the dependence on temperature, in a wide temperature region (namely, a plateau region is exhibited), a resin composition containing (A) an ionomer that includes an ethylene-α,β-unsaturated carboxylic acid copolymer and a ternary polymer of ethylene-α,β-unsaturated carboxylic acid-α,β-unsaturated carboxylic acid ester and (B) a propylene-based polymer, in which the mass of a structural unit derived from the α,β-unsaturated carboxylic acid ester is from 1% by mass to 4.5% by mass of a total mass of the (A) ionomer, is known (see, for example, International Publication WO 2009/145235).
Moreover, a resin composition which exhibits a relatively low seal initiation temperature, realizes a little amount of electrostatic charge accumulation, and exhibits both lock seal performance and peelable seal performance is also known (see, for example, Japanese National Phase Publication No. 2008-500450).